A carbonylation process of a methanol method is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The obtained reaction mixture is separated in an evaporator into a vapor phase containing acetic acid and lower boiling point components and a residual liquid phase containing acetic acid and the catalyst, and the vapor phase is separated by distillation in a distillation column (lower boiling point component removal column) into an overhead stream containing lower boiling point components and an acetic acid stream. The acetic acid stream is further purified to obtain product acetic acid. In this process, acetaldehyde is produced as a by-product during the reaction, and this acetaldehyde is responsible for reducing the quality of the product acetic acid. Therefore, for example, in Patent Literature 1, a condensate of the overhead stream of the lower boiling point component removal column is separated into an aqueous phase and an organic phase using a decanter. Of them, the aqueous phase is distilled in an acetaldehyde removal column, and a condensate (containing acetaldehyde and methyl iodide) of an overhead stream thereof is subjected to extraction with water to thereby separate and remove acetaldehyde.